Electromagnetic radiation, natural and man-made, can often impair the functioning of electrical or electronic equipment. Such radiation can also interfere with the storage of information on computer diskettes. Further, there is a need to provide security against electronic surveillance of, and/or electronic interference with, proprietary electronic data. It is also believed that exposure to radiation of certain wavelengths may have adverse health effects on humans, as well as possibly affecting human behavior patterns.
The transmission by electrical means of information may involve propagation of energy by electromagnetic waves. Electromagnetic waves can be transmitted and received by antennae. An antenna consists of suitably disposed conductors which will radiate electromagnetic energy when connected to a source at an appropriate frequency. When the antenna is being excited, a distribution of current will exist over all the conduction surfaces, which distribution is determined by the conductor geometry and the frequency of the source.
An electromagnetic field at various power frequencies exists in most inhabited areas. This field results from various forms of lighting, power distribution equipment, powered instruments, machinery, and so on.
All efforts in shielding a room are directed towards reducing unwanted electromagnetic signal pickup. Electromagnetic shielding is often required at one or more locations within the electromagnetic field created. A second shield often must be utilized to contain currents caused by, for example, transformer voltages. The number of separate shields desired in a system may depend on the number of independent signals being processed through an electrical device, plus one for each power entrance.
Known methods for rendering objects, such as rooms and electronic machinery, secure against such electromagnetic interference or surveillance include the construction of sheet-metal encased rooms. A second "room-within-a-room" is then constructed with desirable aesthetics. This double wall procedure can be very expensive. Alternatively, electrical transformers can be made to order with special shields at additional costs. A Faraday shield is another shielding or grounding technique and utilizes a layer of copper or aluminum foil laid between electrical coils. Further insulation applied over the foil permits a second shield to cover the first, if required. Faraday shields, however, are only moderately effective, and mutual capacitances to points outside of the shield can be 50 to 100 picoFarad. Wraps of copper or aluminum can be folded to box in the sides of an electrical coil, thereby providing a very tight shield. However, such box shields are labor intensive and expensive, and problems with mutual capacitance, pinhole leakage and shorted turns exist.
Shields can also be made of conductive paper as well as nonferrous metal foil. A paper shield has the advantage of being thin and inexpensive, but may suffer from lack of strength and handleability.
It is also known to use metal foil in wall covering materials. Such materials are especially useful as barriers against the transmission of vapor and/or heat, or in furnishing flame resistance. U.S. Pat. Nos. 4,296,162 and U.S. Pat. No. 4,205,110 disclose thin flexible covering materials which provide heat and vapor barriers for walls or ceilings. The patents teach the use of a layer of vinyl and/or cloth and a metal foil and disclose composites wherein the metal foil is sandwiched between paper or fabric and vinyl. Neither patent teaches electromagnetic shielding.
U.S. Pat. No. 4,647,714 discloses a composite sheet material used for shielding magnetism and electromagnetic waves. However, the composites require electrodeposited iron foil as the core and two metal plating layers. It further teaches that sufficient shielding effects cannot be expected from plastics laminated with aluminum foil.
For electromagnetic radiation shielding it is most desirable to create a continuous electrically conductive barrier around the object. This facilitates the use of a single point ground which is useful in security applications, whereby once one portion of the conductive barrier is grounded, the entire barrier surrounding the object is fully grounded.